In the past, as described in U.S. Pat. No. 3,203,074, a flexible sheet has been held on a rotary carrier by clamping the edge of the sheet between the outer ends of a plurality of leaf springs and a fixed clamping strip. The inner ends of the leaf springs are mounted on a support. In response to the centrifugal force generated by rotation of the carrier, the support moves radially outward. This movement causes the inner ends of the leaf springs to tighten their grip on the edge of the sheet, and to also pull the edge of the sheet along the fixed clamping strip, so as to thereby stretch the sheet on the carrier.
A problem with the prior art clamp is that independent variation of the magnitude of the centrifugally generated grip-tightening force and the sheet-pulling force is not possible. Independent variation of the grip-tightening and sheet-pulling force is highly desirable to accommodate different properties of sheets to be clamped on the rotary carrier.
One type of sheet to be clamped on the carrier may be relatively pliant. Such a sheet requires a small grip-tightening force and a large sheet-pulling force. The grip-tightening force must be small so that the clamp faces will not exert excessive pressure on the edge of the sheet. Excessive pressure can cause permanent deformation of the pliant sheet at the point where the clamp faces grip the sheet. However, the sheet-pulling force must be large enough to take up the pliant sheet's high degree of stretch. If the stretch is not fully taken up, the sheet will not be held tightly as the carrier rotates, and the machine employing the carrier will not function properly. A small grip-tightening force and a large sheet-pulling force is therefore required for a pliant sheet.
Another type of sheet to be clamped on the carrier may be thin and easily torn. Such a sheet requires a large grip-tightening force and a small sheet-pulling force. The grip-tightening force must be large enough to prevent the thin sheet from slipping out of the clamp's grip upon rotation of the carrier. However, the sheet-pulling force must be so small as to prevent the sheet from tearing. A large grip-tightening force and a small sheet-pulling force is therefore required for a thin, easily torn sheet.
Other sheets may have still other properties that require different proportions of grip-tightening and sheet-pulling forces. The ability to independently vary the magnitude of the grip-tightening force and the sheet-pulling force is thus a highly desirable feature to accommodate different properties of sheets.
As discussed hereinabove with respect to the prior art clamp, the sheet is stretched by leaf springs pushing the inner surface of the sheet along a fixed clamping strip. Pushing only the inner surface of the sheet may cause the sheet to tear. This can be avoided by moving cooperating clamp faces (on opposite surface sides of the sheet) concurrently, when stretching the sheet.
Pushing only the inner surface of the sheet may also cause the edge of the sheet to abut against a support of the fixed clamping strip, thereby buckling the sheet's edge. It is desirable to move both cooperating clamp faces concurrently when stretching the sheet to minimize the risk of sheet buckling.